[FIX] mrp_multi_level: readd extensibility hook

_should_create_planned_order hook was unnoticely removed in
33cf4af1ac as it was not needed
anymore in the base module. However it is still an extension point
that can be used.

mrp_multi_level/models/product_mrp_area.py

@@ -313,3 +313,7 @@ class ProductMRPArea(models.Model):
     def _get_locations(self):
         self.ensure_one()
         return self.mrp_area_id._get_locations()
+
+    def _should_create_planned_order(self):
+        self.ensure_one()
+        return True